This application claims the Paris convention priority of Japanese Patent Application No. 11-363692/1999 filed on Dec. 22, 1999, which is incorporated herein by reference.
The present invention relates to a lithium secondary battery using a nonaqueous electrolyte including an electrolytic salt dissolved in a nonaqueous solvent, and more particularly, it relates to improvement of a nonaqueous electrolyte for the purpose of providing a lithium secondary battery exhibiting better charge-discharge cycle performance than a lithium secondary battery using a conventional nonaqueous electrolyte.
A conventional water reactive lithium secondary battery uses, as an electrolyte, a nonaqueous electrolyte including an electrolytic salt dissolved in a nonaqueous solvent.
Examples of the conventionally used nonaqueous solvent are ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, 1,2-dimethoxyethane and a mixed solvent including any of these solvents. An example of the conventionally used electrolytic salt is a lithium salt such as LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2 or LiN(C2F5SO2)2.
In order to improve the characteristics, such as the charge-discharge cycle performance in particular, of a lithium secondary battery, a variety of improvements have been proposed with respect to not only a positive electrode active material and a negative electrode active material but also a nonaqueous electrolyte.
For example, it has been reported that the charge-discharge cycle performance can be improved by using both LIPF6 and LIBF4 as the electrolytic salt of the nonaqueous electrolyte in a lithium secondary battery using LixMO2 (wherein M is one or more transition metals; and 0.05xe2x89xa6xxe2x89xa61.10) as the positive electrode active material (Japanese Laid-Open Patent Publication No. 8-17468/1996).
As a result of examination made by the present inventors, however, it has been found that the charge-discharge cycle performance, at a high temperature of approximately 50 through 60xc2x0 C. in particular, cannot be largely improved by using both LiPF6 and LIBF4. This is probably because Pxe2x80x94F bonds and Bxe2x80x94F bonds with low bonding strength are cut through repeated charge and discharge, resulting in releasing fluorine ions degrading the charge-discharge cycle performance or lowering the concentration of the electrolytic salt in the nonaqueous electrolyte.
Accordingly, an object of the invention is providing a lithium secondary battery exhibiting better charge-discharge cycle performance than a lithium secondary battery using a conventional nonaqueous electrolyte. This object is achieved by using a specific lithium salt as the electrolytic salt of the nonaqueous electrolyte as described in detail below.
The lithium secondary battery of this invention (present battery) comprises a positive electrode, a negative electrode and a nonaqueous electrolyte including an electrolytic salt dissolved in a nonaqueous solvent, and a part or whole of the electrolytic salt is lithium tetrakis(pentafluorophenyl)borate.
As a result, the present battery can exhibit better charge-discharge cycle performance than a lithium secondary battery using a conventional nonaqueous electrolyte.